The action of thrombin on small substrates, platelets and fibrinogen and its interaction with inhibitors, particularly antithrombin-heparin cofactor, will be investigated. Rapid mixing stop-flow methods, conventional kinetic techniques, and electron spin resonance methods will be directed toward a detailed mechanistic understanding of thrombin reactions. BIBLIOGRAPHIC REFERENCES: Sachs, F. and Feinman, R. D. Spin-Labelled Human Platelets. Thrombosis Research, 8, 43 (1976). Feinman, R. D. and Detwilder, T. C. (Letter to the Ed.) Absence of a Requirement for Extracellular Calcium for Secretion from Platelets. Thrombosis Research 1, 677 (1975).